Acquisition Activity - Narrative (Details)		3 Months Ended	12 Months Ended
	May 16, 2016 USD ($) bank $ / shares shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 739,023,000	$ 739,023,000	$ 739,023,000	$ 697,807,000
Acquisition expenses			710,000	15,692,000	0
HF Financial Corp
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%
Equity interest issued per share as part of merger (in shares) | shares	0.65
Consideration paid as part of merger (in dollars per share) | $ / shares	$ 19.50
Percentage of outstanding shares exchanged for cash as part of merger	24.29%
Percentage of outstanding shares exchanged for common stock as part of merger	75.71%
Net purchase price	$ 141,965,000
Goodwill	$ 41,216,000
Number of bank locations acquired | bank	23
Goodwill expected to be tax deductible	$ 0
Intangible assets and loan servicing rights	14,500,000
Increase in goodwill		$ 600,000
Acquisition expenses			$ 700,000	$ 15,700,000	$ 0
Net loans acquired	863,741,000
Acquired loans, estimated discount	$ 28,500,000
Acquired loans, percentage of estimated discount (as a percent)	3.30%
HF Financial Corp | Receivables acquired with deteriorated credit quality
Business Acquisition [Line Items]
Net loans acquired	$ 65,400,000